The present invention relates to a developing roll for use in electrostatic image-developing apparatus employing magnetic particles, and more particularly to such rolls having axially extending magnetic poles of alternating polarity peripherally positioned about its curved surface.
Developers conventionally used for developing latent images on an electrostatic latent image carrier such as a photoconductive body are classified into bi-components developer consisting of two components of magnetic carrier and toner particles, and uni-component developer in which magnetic particles are incorporated in the toner particles.
Also, the developing methods are generally classified into cascade type and magnetic brush type. In the past, the cascade type developing method had been widely used. However, due to the so-called edge effect or fringing effect, which disadvantageously causes an insufficient developing at the central portion of the region to be developed, and due to other disadvantages inherent in the cascade type method, the magnetic brush type developing method has been becoming popular in recent years.
In the developing apparatus of the magnetic brush type, the developer particles are conveyed to the region of a latent image on a carrier, in accordance with the rotation of a developing roll. The developer particles then protrude in a brush-like form, in the area close to the electrostatic latent image, by the attracting force caused by a permanent magnet incorporated in the developing roll. The developer particles are deposited on the latent image to render the latter visible, as the image is rubbed by the brush-like mass of developing particles, as a result of the rotation of the developing roll or of the movement of the image itself.
The developing roll used for this purpose may be a columnar magnet consisting of, as shown in the specification of Anderson's U.S. Pat. No. 3,455,276, a plurality of previously magnetized sector-shaped rubber permanent magnets secured around a rotary shaft, or may be one constituted by block-shaped permanent magnets as disclosed in the specification of U.S. Pat. No. 3,402,698 of Kojima et al.
It has been well-known, however, to those skilled in the art that when the permanent magnet is to be incorporated in the developing roll it is preferable that an integrally formed ceramic permanent magnet be used. Clearly, the axially extending integral columnar magnet can provide an uniform distribution of the magnetic flux on the side line of the magnet, when magnetized, because it has no seam of magnet pieces on the magnetic poles. The use of such a developing roll as incorporating an integral ceramic permanent magnet has been disclosed in the specification of U.S. Pat. No. 3,828,730 of Yamashita et al, and also in the specification of U.S. Pat. No. 3,952,701.
It is extremely difficult to magnetize the ceramic magnet piece formed integrally. Only Japanese Utility Model Laid-open Publication No. 51-14,800 (Published on Feb. 3, 1976) is concerned with this magnetization of the integral ceramic magnet piece. However, this Publication fails to disclose the practical measure for effecting the magnetization.
The present inventors have worked out a magnetizing means capable of practically producing a complicated magnetic pole pattern on a columnar magnet piece for forming a developing roll of increased efficiency.
It is therefore a principal object of the invention to provide magnetizing means capable of magnetizing a columnar permanent magnet so that the latter may have a magnetic pole pattern which would diminish the deterioration of the developer.
To this end, according to the invention, there is provided a developing roll for use in electrostatic image-developing apparatus employing magnetic particles comprising: a shaft, a columnar permanent magnet mounted on the shaft for producing an axially extending uniform magnetic field with peripheral reversals of polarity, and an outer non-magnetic shell surrounding the permanent magnet in a spaced relation therefrom, said shell and said magnet being rotatable relative to each other, the magnet having partially weakened magnetic field portions along its peripheral line for reduction of fatigue of the magnetic particles.
The above and other objects, as well as advantageous features of the invention will become clear from the following description of preferred embodiment taken in conjunction with the attached drawings in which: